Migrate Documentation to terraform-plugin-docs

[bflad]

The "standard library" Terraform Providers should implement nascent provider development tooling to encourage consistency, foster automation where possible, and discover bugs/enhancements to that tooling. To that end, this provider's documentation should be switched to terraform-plugin-docs, including:

• Migrating directory structures and files as necessary
• Enabling automation for documentation generation (e.g. make gen or go generate ./...)
• Enabling automated checking that documentation has been re-generated during pull request testing (e.g. no differences)